The present invention relates to a tensioning device for the drive spring of an electrical switch, the device including a drive spring articulated to the rotating crank of a drive shaft.
Tensioning devices of this type have been known for a long time and are used to great benefit in commercially available switches such as, for example, the type VA three-pole vacuum-type circuit breaker manufactured by Sachenwerk Aktiengesellschaft, Federal Republic of Germany. The drive spring here generally stores a certain amount of energy which is sufficient for turn-on as well as for charging the turn-off spring of the power switch. A low power electric motor or a manual drive tensions the drive spring, with directional couplings, usually in the form of free-wheeling couplings, acting as step-down gears. These gears impart stepwise advances and act as a block against reversal of the tensioning movement of the drive spring.
Due to the mass inertia of the rotating parts, tensioning devices for electrical switches, when the devices are actuated by motor drives by way of directional couplings, exert an undesirable additional load on the blocking latch of the turn-on lock after the motor circuit has been interrupted. Such additional loads may also occur in manually actuated tensioning devices.
European Pat. No. 0,174,906, and its counterpart U.S. Pat. No. 4,655,098, solves the respective problem in a tensioning device operated by way of a flexible pulling means by using a free-wheeling clamping roller in which, at the end of the tensioning process, the clamping rolls are brought out of engagement with the aid of a multi-part lever arrangement disposed at the outer ring. So-called one-turn free-wheeling couplings, as disclosed, for example, at pages 71-75 of the catalog entitled "Stieler Prazision" published by the firm Walther Flender, may be used. These machine parts fulfill the requirements they are intended to meet but constitute a very expensive and space consuming solution because the multi-part lever arrangement must be provided at each free-wheeling device and because such a free-wheeling device has a significantly greater width than a simple free-wheeling device.
Brochure No. SR 26,620 dated September, 1976, by the firm Grady of Geneva, Switzerland, entitled Olarmer Leistungsschalter GUD f u/ r Innenmontage [Low-Oil Power Switch GUD for Internal Installation]discloses a drive in which a motor equipped with an eccentric and drive levers attached to the outer ring of a free-wheeling device causes a tensioning shaft to perform a stepwise rotary movement, while a second free-wheeling device supported at the drive housing serves as the reversal block. A pinion transfers the rotation of the tensioning shaft to a revolving gear at which is articulated the drive spring of the power switch as well as a bolt with which, at the end of the tensioning movement, the drive lever is brought out of engagement. The details are shown in the "Anleitung fur die Inbetriebsetzung . . . " [Instructions for Start-Up Operation] for the above-mentioned power switch, Chapter SR, Sheet 2661/2. In this drive, the hand crank acts directly on the tensioning shaft, without a possibility of unlatching, and may lead to jamming of the turn-on lock.
The rotary pulse for uncoupling the drive lever is furnished in the prior art tensioning devices by the drive spring on its path between the dead center position in the tensioned state and the support in the turn-on lock. The available energy must here also overcome the force of the spring with which the drive lever is guided along the eccentric of the motor. These requirements place high demands on the setting of the drives in the testing field because, on the one hand, sufficient energy must be available to decouple the drive lever even with increasing friction due to longer periods of use and/or low ambient temperatures while, on the other hand, no sudden high energy loads are desired on the support in the turn-on lock. Self-locking must therefore be avoided in the kinematic design of the tensioning device.